To date, various types of optical memories have been proposed as optical memories with which information can be reproduced by irradiating a light beam onto an information recording medium and detecting the light that is reflected. These include ROM (Read Only Memory) type memories in which information is recorded by phase pits, write-once type optical memories in which information is recorded by forming holes in a recording film by irradiation of a light beam, phase-change type optical memories in which recording is performed by changing the crystalline phase of a recording film by irradiation of a light beam, and magneto-optical memories in which recording is performed by changing the direction of magnetization of a recording layer by irradiating a light beam and applying a magnetic field.
In these optical memories, the reproduction resolution of signals is determined almost entirely by the wavelength λ of the reproduction light beam and the numerical aperture (N.A.) of the objective lens, and the pit period at the detection limit was substantially λ/(2·N.A.). However, since it is not easy to shorten the wavelength of the reproduction light beam or to increase the numerical aperture of the objective lens, attempts to increase the recording density of information by modifying the recording medium or the reproduction method have been undertaken. In particular, various attempts to increase the recording density of information in magneto-optical recording media have been proposed. For example, JP H6-290496A proposes a technology for increasing the reproduction resolution by exceeding the detection limit that is determined by the wavelength and the numerical aperture of the objective lens as mentioned above by successively moving magnetic domain walls that have approached the reproduction light beam and detecting the movement of these magnetic domain walls. With this technology, a particularly good reproduction signal can be obtained when the reproduction layer, which is a first magnetic layer in which magnetic domain walls are moved when they have approached the reproduction light beam, is magnetically separated between information tracks.
However, the reproduction of recorded information by transferring tiny recording magnetic domains that have been recorded at high density on a recording layer to a reproduction layer and moving the magnetic domain walls of the reproduction layer, for example, requires that the tiny magnetic domains of the recording layer are kept stable and are very stably transferred to the reproduction layer through magnetic coupling. In particular, there is the problem that the vertical magnetic anisotropy of the recording layer becomes small depending on the composition or manufacturing method of the recording film, and thus it is difficult to form tiny recording magnetic domains stably. Moreover, stable magnetic coupling that employs the vertical magnetic anisotropy of the recording layer is necessary in order to transfer the recording magnetic domains of the recording layer to the reproduction layer, and there is the problem that if transfer is not stable due to changes in the transferability related to the magnetic properties of the recording layer, then there is an increase in transfer noise and noise that accompanies magnetic domain wall movement, and this lowers the quality of the reproduced signal.
Further, to carry out magnetic domain wall movement stably, methods such as magnetically separating information tracks by laser annealing or using an optical disk substrate having a land/groove structure to isolate information tracks from one another have been adopted, but there is the problem that the transferability from the recording layer to the reproduction layer changes depending on the conditions of the laser annealing or the groove shape of the lands and grooves of the optical disk substrate, and that the effect of the groove noise from the optical disk substrate is large. In particular, there is the problem that if grooves are deep or narrow, then when recording to grooves there is a drop in magnetic coupling, such as the exchange coupling force, between the recording layer and the reproduction layer.